The present invention pertains to mandrel systems for receiving a hollow cup-like product for printing on the exterior surface of the product side wall.
In a typical prior art mandrel system, the hollow cup-like product is received on a mandrel and rotated by the mandrel against one or more print rollers. The print rollers have a soft exterior pad of a material, such as soft rubber, to which imaging material, such as paint or ink, is applied to predetermined areas and held to define the mirror image of a desired image. The desired image is applied to the exterior surface of the side wall of the cup-like product when the print roller and the product are engaged and rotated to transfer the imaging material from the print roller to the product.
The all of the mandrel includes apertures. As the product is being received by the mandrel, a vacuum is created inside the mandrel, whereby air is sucked into the mandrel through the apertures to cause the product to be drawn into firm engagement with the mandrel. After the mandrel has been rotated in order to rotate the product against the print rollers, the air flow within the mandrel is reversed so that air flows out through the apertures in the mandrel wall to thereby force the product from the mandrel.
The exterior surface of the mandrel wall that engages the interior surface of the side wall of the cup-like product is smooth, except for the apertures, in order to match the smooth interior-surface contour of a typical cup-like product side wall. When the interior surface of the product side wall is not smooth, however, such as when the interior surface of the product side wall includes ribs, certain problems arise. The absence of a uniformly smooth engagement between the mandrel and the interior surface of the product side wall necessitates the application of a greater vacuum force to hold the product in firm engagement with the mandrel so that the product will not slip while being rotated by the mandrel against the print rollers. Also, if the product has a thin side wall that is somewhat flexible, the force applied to the side wall of the product by the print rollers during their engagement with the product may bend the product in the regions between the ribs and thereby cause an inexact transfer of the image from the print rollers to the exterior surface of the product side wall.